Problem: Which of these numbers is prime? ${9,\ 17,\ 25,\ 45,\ 78}$
Solution: A prime number has exactly two factors: 1 and itself. The factors of 9 are 1, 3, and 9. The factors of 17 are 1 and 17. The factors of 25 are 1, 5, and 25. The factors of 45 are 1, 3, 5, 9, 15, and 45. The factors of 78 are 1, 2, 3, 6, 13, 26, 39, and 78. Thus, 17 is a prime number.